A solar cell or PV device takes advantage of the material property of semiconductors to convert light energy to electricity. It does this by creating free charge carriers through the absorption of photons. In general terms, the photon cannot be absorbed if its energy is lower than the band gap energy. On the other hand, if the photon has more energy than the band gap energy, the excess energy will be lost in the form of heat. Therefore, only a particular part of solar spectrum can be well absorbed for a chosen semiconductor. The type of solar cell in which only one p-n or p-i-n junction is used is known as single junction solar cell.
The efficiency of a single junction solar cell can be improved by introducing more p-n or p-i-n junctions into the device. In such cell, several sub-cells made of materials of different photoelectric properties are stacked on top of each other. Such cells are known generally as multi-junction solar cells, or specifically, in the case of a cell comprising two or three p-n or p-i-n junctions, as a tandem or triple junction solar cell, respectively. Each sub-cell absorbs a different part of the solar spectrum, hence the overall absorption of the solar cell is increased.
The efficiency of a single junction solar cell can also be improved by using vertical p-n or p-i-n junction. The vertically aligned junction decouples the light absorption depth from the carrier extraction length; therefore more light can be absorbed while simultaneously maintaining high carrier extraction efficiency (FIG. 2). For more detailed explanation, reference is made to “Nanostructured Substrate for Rapid Thin Film growth” (British Patent Application No 0813568.3)
The vertical junction can be formed by first creating scaffolding structures, where the scaffolding structures can be substantially vertical pillars or fins, followed by deposition of an appropriate combination of layers of thin films around the scaffolding structures to form the desired junctions. The scaffolding structures can be made of a conductor, or an insulator or a semiconductor. Either additive or subtractive methods can be applied to form the vertical scaffolding structure on the substrate,
Both multi-junctions and vertical junctions offers opportunities to improve the PV device efficiency, it is hence desirable to take advantage of both methods and incorporate vertical junctions into each of the sub-cells of a multi-junction PV device (FIG. 3). By boosting the efficiency of each sub-cell, it will increase the overall performance of a multi-junction PV device even further.
Although incorporating vertical p-n or p-i-n junctions in each sub-cell of the PV device will result in an enhancement of cell efficiency, manufacture of such a structured device is not straight forward; for example, such a process may require forming a set of vertical structures on top of the previous sub-cell for the subsequent structured sub-cell to be formed (FIG. 4). This will introduce cost and complexity into the manufacturing process, which partially or completely offset the value added through gains in efficiency.
This invention enables the formation of vertical junctions in each sub-cell in a cost effective way by introduction of a novel structured substrate. The features on the structured substrate are only formed once before the material for each sub-cell is deposited, and their profile enables the formation of vertical junctions in at least two sub-cells of a multi-junction device.